1. Field of the Invention
This invention relates generally to the field of optical and medical devices, and more specifically to an apparatus and method for on-chip optical spectroscopy for the detection and identification by unique spectral signatures of solid, liquid, or gas substances using photonic crystals.
2. Background of the Invention
Defect engineered photonic crystals, with sub micron dimensions have demonstrated high sensitivity to trace volumes of analytes for performing a large range of sensing applications. Photonic crystals have been described and discussed by Joannopoulos, J. D., R. D. Meade, and J. N. Winn, in Photonic Crystals, 1995 Princeton, N.J.: Princeton University Press. However, exact identification of analyte through spectroscopic signatures has not been demonstrated. Furthermore, much of the research in photonic crystal devices has relied on enhancing refractive index sensitivity to a single analyte (see Lee M. R., and Fauchet M., “Nanoscale microcavity sensor for single particle detection,” Optics Letters 32, 3284 (2007)) and detection of a single analyte. Some of the popular commercially available optical spectroscopy techniques are cavity ringdown spectroscopy (CRDS) described by Thorpe M. J. et al., in “Broadband cavity ringdown spectroscopy for sensitive and rapid molecular detection,” Science 311, 1595 (2006), and tunable diode laser absorption spectroscopy (TDLAS) described by Druy M., in “From laboratory technique to process gas sensor: the maturation of tunable diode laser absorption spectroscopy”, in Spectroscopy 21(3), 14 (2006). Cavity ringdown spectroscopy cannot be integrated on-chip primarily due to the stringent requirement of optical source and high finesse optical cavities for analyte sampling. Technically, tunable diode laser absorption spectroscopy could be integrated on-chip, but the analyte sampling volume required is of the order of meters and cannot therefore be integrated on a chip. Hence, tunable diode laser absorption spectroscopy is also not a convenient method to integrate on-chip. Furthermore, both cavity ringdown spectroscopy, due to the requirement of femtosecond lasers and high finesse optical cavities for high sensitivity, and tunable diode laser absorption spectroscopy, are very expensive, of the order of tens of thousands of dollars. A primary drawback of both cavity ringdown spectroscopy and tunable diode laser absorption spectroscopy is the weight, size, as well as cost which significantly increases the cost of ownership of the corresponding products. The same drawbacks of size, weight, and cost hold for Fourier transform infrared spectroscopy and photoacoustic spectroscopy. A lab-on-chip integrated infrared spectrometer for remote, in-situ sensing and spectroscopic identification is highly desired for widespread deployment to enhance the detection of hazardous pollutants for environmental and homeland security. Research on on-chip spectroscopy has resulted in methods such as a ring resonator based microspectrometer described by Kyotoku B. B. C., Chen L., and Lipson M., in “Sub-nm resolution cavity enhanced microspectrometer,” in Optics Express 18(1), 102 (2010) for optical spectral analysis, by Robinson J. T., Chen L., and Lipson M., in “On-chip gas detection in silicon optical microcavities,” in Optics Express 16(6), 4296 (2008) for gas sensing and photonic crystal microcavity-based devices described by Canon Kabushiki Kaisha in US Patent Application 20060285114, “Gas detection and photonic crystal devices design using predicted spectral responses” which rely on the quality factor of the optical microcavity to trap light and enhance the optical interaction with the analyte by increasing the effective interaction time with the analyte with photonic crystal microcavities. However, these devices are limited by resolution of spectrometry. In order to increase the resolution of the spectrometer, either the size of the ring resonator must be increased, the number of photonic crystal microcavities must be increased in proportion to the resolution, or complex grating structures need to be fabricated as described by Kyotoku B. B. C., Chen L., and Lipson M., in “Sub-nm resolution cavity enhanced microspectrometer,” in Optics Express 18(1), 102 (2010). Research has been performed with hollow core waveguides for atomic spectroscopy on-chip by Yang W., Conkey D. B, Wu B., Yin D., Hawkins A. R., and Schmidt H., in “Atomic spectroscopy on a chip” in Nature Photonics 1, 331 (2007), which are limited to large sizes when the absorption cross-section of the analyte becomes small.
Two dimensional photonic crystal waveguides integrated with slot waveguides offer the possibility of integrating the slow light effect of two-dimensional photonic crystal waveguides with the optical field enhancement effect of slot waveguides to enhance the optical interaction between light and analyte. According to Beer-Lambert absorption technique, transmitted intensity I is given by equation 1 asI=I0exp(−γAL)  (1)where I0 is the incident intensity, A is the absorption coefficient of the medium, L is the interaction length, and γ is the medium-specific absorption factor determined by dispersion enhanced light-matter interaction. In conventional systems, L must be large to achieve a suitable sensitivity of the measured I/I0. For lab-on-chip systems, L must be small, hence γ must be large. Mortensen et al. showed [Mortensen N. A., Xiao S. S., “Slow-light enhancement of Beer-Lambert-Bouguer absorption,” Applied Physics Letters 90 (14), 141108 (2007)] using perturbation theory that
                    γ        =                  f          ×                                    c              /              n                                      v              g                                                          (        2        )            
where c is the velocity of light in free space, vg is the group velocity in the medium of effective index n, and f is the filling factor denoting the relative fraction of the optical field residing in the analyte medium. Equation 2 shows that slow light propagation (small vg) significantly enhances absorption. Furthermore, the greater the electric field overlap with the analyte, the greater the effective absorption by the medium. In a conventional waveguide, the optical mode interacts with the analyte only through its evanescent tail. In a slot waveguide, the guided optical mode not only interacts with the analyte environment with its evanescent tail, but also interacts with the enhanced optical field intensity in the slot. In a photonic crystal waveguide as theoretically proposed by Mortensen, only the group velocity vg is reduced. By introducing a slot in a two-dimensional photonic crystal waveguide, we have demonstrated experimentally that in a two-dimensional photonic crystal slot waveguide, the group velocity is reduced by a factor of 100 due to the slow light effect and the optical field intensity is increased by a factor of 10 in a slot compared to evanescent guiding only. As a result, the effective absorption length is increased by a factor of 1000 compared to the geometrical optical length, which increases the optical absorption by the analyte, as determined by the Beer-Lambert law of optical absorption. The factor of 1000 is much larger than the factor of 10 demonstrated by Jensen et al. using a one-dimensional Bragg stack that employs group velocity vg reduction only and has a much smaller slow light effect due to one-dimensional confinement of the slow light propagating optical mode [Jensen K. H., Alam M. N., Scherer B., Lambrecht A., Mortensen N. A. “Slow light enhanced light-matter interactions with applications to gas sensing”, Optics Communications 281 (21), 5335 (2008)]. The factor of 1000 improvement in our photonic crystal slot waveguide spectrometer due to the combined effects of high group velocity enhancement and optical intensity enhancement in a two-dimensional photonic crystal slot waveguide results in absorption lengths of the order of hundreds of microns on-chip compared to tens of centimeters in single pass off-chip spectroscopy techniques.
Spectrometer techniques such as cavity ring-down spectroscopy, ring resonator optical cavity, and photonic crystal cavity are limited in resolution, by the finesse of the optical cavity and the size and weight of the optical cavity. Designs are needed to make a miniaturized, light-weight on-chip integrated spectrometer that can measure a continuous absorption spectrum, not limited by resolution of the spectrometer.
Designs are therefore needed in the art to integrate two-dimensional photonic crystal waveguides with slot waveguides for optical absorption spectroscopy on-chip.